ETOPO, GTOPO, NIMA... oceans and Antarctica uncolored... I use for grid reference...Contour colors are in rainbow-geographic order. Selection of the contour interval to display 'breaks' in elevation depend on the selected 'use'. Scale, resolution, file format, display format (DEM or TIN), accuracy, metadata, history, and use are all always in debate. They are all tools to be used in communication. Simply changing the contour interval will alter the terrain image and possibly allow data processing artifacts to skew the view. Contour intervals are often chosen to be the same as the source data, 1000 meter 'weather breaks', 'climate' breaks, military capability - temperature breaks, selected range enhanced, or 'artistic'.

The following excerpts are from many sources...

USGS Digital Elevation Model (DEM) data files

Descriptive names refer to the size of the cell, and/or the size of the data point. 1' is 1 Minute.

7.5':1AS=30 Meters, 30': 2AS, 1 Degree=60': 3AS, 30 AS, 5 Minute is 300AS. My calculations indicate the major forms of data are 1,2,3,30, or 300 Arc Seconds per data point.

{1 AS=30 meters <?> 30AS = 1000 meters}

The USGS DEM data for 7.5-minute units correspond to the USGS 1:24,000 and 1:25,000 scale topographic quadrangle map series for all of the United States and its territories. Each 7.5-minute DEM is based on 30- by 30-meter data spacing with the Universal Transverse Mercator (UTM) projection. Each 7.5- by 7.5-minute block provides the same coverage as the standard USGS 7.5-minute map series. 7.5-minute 30 meters. The spacing between elevations along profiles is 1 arc second in latitude by 2 arc seconds of longitude

2-arc-second (aka 30 Minute) DEM data cover 30-minute by 30-minute areas. Each cell corresponds to the east half or west half of the USGS 30- by 60-minute maps. Spacing of the elevations along each profile is 2 arc seconds. The first and last data points are at the integer 15 minutes of latitude. A 15-minute profile will, therefore, contain 451 elevations.

The 1-degree DEMs are also referred to as 3-arc-second or 1:250,000 scale DEM data. The 1-degree DEM (3- by 3-arc-second data spacing) provides coverage in 1- by 1-degree cells.

A USGS DEM file is organized into three logical records:

Type A contains information defining the general characteristics of the DEM, including DEM name, boundaries, units of measurement, minimum and maximum elevations, projection parameters, and number of type B records. Each DEM file has one Type A record.

Type B contains profiles of elevation data and associated header information. Each profile has a Type B record. The data are ordered from south to north in profiles that are ordered from west to east. Lower Left to Upper left, by left to right: Columns by row.

Type C contains statistics on the accuracy of the data.

The UTM-based DEM's (7.5-minute DEM) and the geographic-based DEM's (30-minute, Alaska, and 1-degree DEM's) are identical in logical data structure but differ in sampling interval, geographic reference system, areas covered, and horizontal and vertical accuracy. Knowledge of all of these properties is essential to ensure that the user does not exceed the useful limits of the data for required applications.

(USGS) GTOPO30: 30 arc second (nominally one kilometer).

Elevations in GTOPO30 are regularly spaced at 30-arc seconds (approximately 1 kilometer).

The horizontal grid spacing is 30-arc seconds (0.008333333333333 degrees), resulting in a DEM having dimensions of 21,600 rows and 43,200 columns. The data are stored in row major order (all the data for row 1, followed by all the data for row 2, etc.). {Data is 'normal' integers so the bytes must read, 'swapped', then set as integers on PC/Dec-Vax computers.} The files for each cell are: DEM digital elevation model data, HDR header file for DEM, DMW world file, STX statistics file, PRJ projection information file, GIF shaded relief image, SRC source map, SCH header file for source map. The global 16-bit DEM (21,600 rows by 43,200 columns) has a size of 1.74 gigabytes. (size of the global data set is almost 2.72 gigabytes {SRC files are ½ the size as their DEM} {all other files total ~ 1 meg per cell}). The horizontal coordinate system is decimal degrees of latitude and longitude referenced to WGS84. The 27 tiles that individually cover 50 degrees of latitude and 40 degrees of longitude each have 6,000 rows and 4,800 columns. Antarctica is covered by 6 tiles, with each tile covering 30 degrees of latitude and 60 degrees of longitude.

GTOPO30, completed in late 1996, was developed over a 3 year period through a collaborative effort led by staff at the U.S. Geological Survey's EROS Data Center (EDC). The following organizations participated by contributing funding or source data: the National Aeronautics and Space Administration (NASA), the United Nations Environment Programme/Global Resource Information Database (UNEP/GRID), the U.S. Agency for International Development (USAID), the Instituto Nacional de Estadistica Geografica e Informatica (INEGI) of Mexico, the Geographical Survey Institute (GSI) of Japan, Manaaki Whenua Landcare Research of New Zealand, and the Scientific Committee on Antarctic Research (SCAR).

(NOAA) ETOPO5: 5-minute latitude/longitude grid

ETOPO5 was generated from a digital data base of land and sea- floor elevations on a 5-minute latitude/longitude grid. ETOPO5 : The data file is formatted as 16-bit BINARY INTEGERS in two byte orders: ETOPO5.DOS is in IBM-PC/DEC-VAX "swapped," lo-byte-first order. The file ETOPO5.DAT is in "normal," or hi-byte-first order, as used by Macintosh, Sun, and some other workstations. The only "header" file is displayed information so SAVE the text with the same name as the .BIN for input into a display program.

The file may be thought of as having a logical record size of 8640 bytes. The data start at the North Pole (90 deg N, 0 deg 0' E) and are arranged in bands of 360 degrees x 12 points/degree = 4320 values (8640 bytes) ranging eastward from 0 deg 0' East longitude to 359 deg 55' East longitude (since it represents the North Pole, all possible longitudes still refer to a single point, thus the first band has 4320 identical values of -4290 m). The 8641st starts the latitude band for 89 deg 55' N, and so on. There is NO record for the South Pole (elevation 2810 m.). Example of ½ E-W cell: In both files, there are 2160x4320 data values, one for each five minutes of latitude and longitude, for a total of 9,331,200 points or 18,662,400 bytes. Example of my thoughtless 1/4 cell: 2161 x 1081 matrix of 5-minute grid values beginning at NorthWest corner progressing Eastward for 2161 values, then stepping 5 minutes South for the next row, ending at SouthEast corner. File size: 4672082 bytes. (Each has 1 row and 1 column of another cell due to 0 to 180, 180 to 360, 90 to 0, 0 to 90...)

The ETOPO5 data may be credited in publications by reference to "Data Announcement 88-MGG-02, Digital relief of the Surface of the Earth. NOAA, National Geophysical Data Center, Boulder, Colorado, 1988." The version of the data making up ETOPO5 is from May, 1988, with the exception of a small area in Canada (120-130 W, 65-70 N), which was regridded in 1990.

In support of military applications, the National Imagery and Mapping Agency (NIMA) has developed a standard digital dataset (Digital Terrain Elevation Data (DTED®) Level 0) which may be of value to scientific, technical, and other communities . This DTED® product is a uniform matrix of terrain elevation values which provides basic quantitative data for systems and applications that require terrain elevation, slope, and/or surface roughness information. DTED® Level 0 elevation post spacing is 30 arc second (nominally one kilometer). In addition to this discrete elevation file, a separate binary file provides the minimum, maximum, and mean elevation values computed in 30 arc second square areas (organized by one degree cell). Finally, DTED® Level 0 contains the NIMA Digital Mean Elevation Data (DMED) providing minimum, maximum, and mean elevation values and standard deviation for each 15 minute by 15 minute area in a one degree cell.

This data set may be freely copied, manipulated, adapted or combined with other geospatial information as desired by the user. It allows a gross representation of the Earth's surface for general modeling and assessment activities. Such reduced resolution data is not intended and should not be used for automated flight guidance or other precision activity involving the safety of the public. Public domain software capable of displaying, manipulating and analyzing such data will be available through the U.S. Geological Survey, Branch of Information Services, Box 25286, Denver, Colorado 80225 (1-800-435-7627 or 1-800-HELP-MAP).

The USGS Digital Elevation Model (DEM) data files are digital representations of cartographic information in a raster form. DEMs consist of a sampled array of elevations for a number of ground positions at regularly spaced intervals. These digital cartographic/geographic data files are produced by the U.S. Geological Survey (USGS) as part of the National Mapping Program and are sold in 7.5-minute, 15-minute, 2-arc-second (also known as 30-minute), and 1-degree units. The 7.5- and 15-minute DEMs are included in the large scale category while 2-arc-second DEMs fall within the intermediate scale category and 1-degree DEMs fall within the small scale category.

Large scale

The DEM data for 7.5-minute units correspond to the USGS 1:24,000 and 1:25,000 scale topographic quadrangle map series for all of the United States and its territories. Each 7.5-minute DEM is based on 30- by 30-meter data spacing with the Universal Transverse Mercator (UTM) projection. Each 7.5- by 7.5-minute block provides the same coverage as the standard USGS 7.5-minute map series.

The 7.5-minute Alaska DEM data correspond to the USGS 1:24,000 and 1:25,000 scale topographic quadrangle map series of Alaska by unit size. The unit sizes in Alaska vary depending on the latitudinal location of the unit. The 7.5-minute Alaska DEM data consist of a regular array of elevations referenced horizontally to the geographic (latitude/longitude) coordinate system of the North American 1927 Datum (NAD 27) or the North American 1983 Datum (NAD 83). The spacing between elevations along profiles is 1 arc second in latitude by 2 arc seconds of longitude.

The 7.5-minute DEM data are stored as profiles in which the spacing of the elevations along and between each profile is 30 meters. The number of elevations in a profile will vary because of the variable angle between the quadrangle's geographic boundary (neatline) and the UTM coordinate system. DEM data of low-relief terrain or generated from contour maps with intervals of 10 feet or less are recorded in feet while DEM data of moderate to high-relief terrain or generated from maps with terrain contour intervals greater than 10 feet are generally recorded in meters.

The 15-minute DEM data correspond to the USGS 1:63,360 scale topographic quadrangle map series of Alaska by unit size. The unit sizes in Alaska vary depending on the latitudinal location of the unit. The 15-minute DEM data consist of a regular array of elevation referenced horizontally to the geographic (latitude/longitude) coordinate system of NAD 27. The spacing between elevations along profiles is 2 arc seconds of latitude by 3 arc seconds of longitude.

Intermediate scale

The 2-arc-second DEM data cover 30-minute by 30-minute areas which correspond to the east half or west half of the USGS 30- by 60-minute topographic quadrangle map series for the conterminous United States and Hawaii. Each 2-arc-second unit is produced and distributed as four 15- by 15-minute cells. The spacing of elevations along and between each profile is 2 arc seconds.

The 15-minute DEM data are collected with a 2- by 3-arc-second spacing in latitude, and longitude, respectively. The first and last data points along a profile are at the integer degrees of latitude. Elevation data on the quadrangle neatlines (all four sides) share edge profiles with the surrounding eight quadrangles.

Spacing of the elevations along each profile is 2 arc seconds. The first and last data points are at the integer 15 minutes of latitude. A 15-minute profile will, therefore, contain 451 elevations.

Small scale

The 1-degree DEM (3- by 3-arc-second data spacing) provides coverage in 1- by 1-degree blocks for all of the contiguous United States, Hawaii, and most of Alaska. The basic elevation model is produced by or for the Defense Mapping Agency (DMA), but is distributed by the USGS, in DEM data record format. In reformatting the product, the USGS does not change the basic elevation information. The 1-degree DEMs are also referred to as 3-arc-second or 1:250,000 scale DEM data.

Spacing of elevations along and between each profile of 1-degree DEMs is 3 arc seconds with 1,201 elevations per profile. The exception is DEM data in Alaska, where the profile spacing varies depending on the latitudinal location of the DEM. Latitudes between 50 and 70 degrees north have spacings at 6 arc seconds with 601 profiles per 1-degree unit and latitudes greater than 70 degrees north have spacings at 9 arc seconds with 401 profiles per 1-degree unit.

The 1-degree mosaic data set spacing of elevation and profile data is the same as the data of conterminous U.S. 1-degree DEM data set.

DEM Accuracy

The method of determining DEM accuracy involves computation of the root-mean-square error (RMSE) for linearly interpolated elevations in the DEM and corresponding true elevations from the published maps. Test points are well distributed and representative of the terrain. Collection of test point data and comparison of the DEM to the quadrangle hypsography are conducted by USGS quality-control groups.

Large scale

The vertical accuracy of 7.5-minute DEMs is equal to or better than 15 meters. A minimum of 28 test points per DEM is required (20 interior points and 8 edge points). The accuracy of the 7.5-minute DEM data, together with the data spacing, adequately support computer applications that analyze hypsographic features to a level of detail similar to manual interpretations of information as printed at map scales not larger than 1:24,000 scale. Accuracy of the 15-minute DEMs is equal to or better than one-half of a contour interval of the 15-minute topographic quadrangle map. The accuracy of the 15-minute DEM data, together with the data spacing, adequately support computer applications that analyze hypsographic features to a level of detail similar to manual interpretations of information as printed at map scales not larger than 1:63,360 scale. The plotting of contours from the 15-minute Alaska DEMs at scales larger than 1:63,360, or reliance on the elevation heights without incorporating the National Map Accuracy Standard (NMAS) horizontal error tolerance, will lead to less reliable results.

Intermediate scale

The 2-arc-second DEM accuracy is equal to or better than one-half of a contour interval of the 30- by 60-minute topographic quadrangle map. The accuracy of the 2-arc-second DEM data, together with the data spacing, adequately support computer applications that analyze hypsographic features to a level of detail similar to manual interpretations of information as printed at map scales not larger than 1:100,000 scale. The plotting of contours from 2-arc-second DEM data at scales larger than 1:100,000, or reliance on the elevation heights without incorporating the NMAS horizontal error tolerance, will lead to less reliable results.

Small scale

The 1-degree mosaic data set spacing of elevation and profile data is the same as the 1-degree DEM contiguous U.S. data. The accuracy of the 1-degree DEM data, together with the data spacing, adequately support computer applications that analyze hypsographic features to a level of detail similar to manual interpretations of information as printed at map scales not larger than 1:250,000 scale. The plotting of contours from the 1-degree DEM at scales larger than 1:250,000, or reliance on the elevation heights without incorporating the NMAS horizontal error tolerance, will lead to less reliable results.

For general information on accuracy, see Horizontal Positional Accuracy and Vertical Positional Accuracy. Data Organization

A DEM file is organized into three logical records:

Type A

contains information defining the general characteristics of the DEM, including DEM name, boundaries, units of measurement, minimum and maximum elevations, projection parameters, and number of type B records. Each DEM file has one Type A record.

Type B

contains profiles of elevation data and associated header information. Each profile has a Type B record.

Type C

contains statistics on the accuracy of the data.

Examples of a DEM record format may be found in the Appendix.

Entity and Attribute Information

A digital elevation model is composed of integer values representing a gridded form of a topographic map hypsography overlay.

Additional information on DEM specifications can be found in the USGS National Mapping Program's Standards for Digital Elevation Model (DEMs) and in the USGS publication titled "Digital Elevation Models -- Data Users Guide 5."

Classification Levels

DEM data are organized in three classification levels. Level-1 DEMs are elevation data sets in a standardized format. The intent is to reserve this level for 7.5-minute DEMs which are created by scanning National High Altitude Photography (NHAP)/NAPP photography. A vertical RMSE of 7 meters is the desired accuracy standard. A RMSE of 15 meters is the maximum permitted.

Level-2 DEMs are elevation data sets that have been processed or smoothed for consistency and edited to remove identifiable systematic errors. DEM data derived from hyposographic and hydrographic data digitizing, either photogrammetrically or from existing maps, are entered into the Level-2 category. A RMSE of one-half contour interval is the maximum permitted.

Level-3 DEMs are derived from DLG data by incorporating selected elements from both hypsography (contours, spot elevations) and hydrography (lakes, shorelines, drainage). A RMSE of one-third of the contour interval is the maximum permitted.

Digital Elevation Model Caveats

Large scale

The majority of the 7.5-minute DEMs produced to date are categorized as Level-1 DEMs.

Intermediate scale

All 2-arc-second DEMs derived from contours are Level 2. All 2-arc-second DEMs derived for 7.5-minute DEMs are Level 1.

Small scale

All 1-degree DMA DTED-1 data have been classified as Level 3. A more detailed description of the DEM accuracy and classification levels may be found in the USGS publication titled "Digital Elevation Models -- Data Users Guide 5."

Applications and Related Data Sets

The DEM files may be used in the generation of graphics such as isometric projections displaying slope, direction of slope (aspect), and terrain profiles between designated points. They may also be combined with other data types such as stream location data and weather data to assist in forest fire control or they may be combined with remote sensing data to aid in the classification of vegetation. Applications include: (1) modeling terrain gravity data for use in locating energy resources, (2) determining the volume of proposed reservoirs, (3) calculating the amount of material removed during strip mining, (4) determining landslide probability, and (5) developing parameters for hydrologic models.